Why can't Apple allow individuals who need to physically access equipment to go on campus on their own, be in the room by themselves, without demanding that they be exposed to groups of other people for days?

If just one person is ill, then many others will get ill in an office environment.

But scheduling a person to use equipment and having an hour between people to circulate the air properly should be doable. Meanwhile, the vast majority of Apple engineers can obviously work from home, so why not let them if they choose to do so?